Direction of 180°-Pulse

Diagrams always seem to show the 180° pulse flipping the spins over the top right side of the transverse plane. Why don't the spins flip 180° in some other direction, for example, below the transverse plane and to the left?

Small technical points such as this frequently bewilder the beginning student but are seldom explained in textbooks on clinical MR imaging. Indeed, both types of 180° flipping are possible; which type of flip occurs in a given situation depends on the phase of the transmitted RF pulse.

Recall that an "RF pulse" is merely a magnetic field (B1) of short duration rotating at the Larmor frequency in the transverse plane. Depending on its phase, B1 may be considered to be "directed" along the x-axis or y-axis (or anywhere else in between). The above illustration shows the different refocusing directions produced by 180°-pulses along different axes.

By adjusting the phase of transmission, the 180° pulse may be applied along the x-axis, y-axis, or any other direction. On the left side of the diagram is a 180y° pulse, that is, one that rotates the spins around the y-axis. The echo forms in the +y-direction. On the right is a 180x° pulse that causes the echo to form along the −y-direction.

In the early SE experiments by Hahn (1950) and Carr and Purcell (1954), RF pulses were all applied along the same axis (usually x-direction). In practice, this method resulted in measured T2 values that were too short because of (1) cumulative phase errors from repetitive imperfect 180° pulses, and (2) B1 inhomogeneity effects that spread the magnetization out in a plane containing B1 and Bo. In 1958 Meiboom and Gill proposed that such pulse-related errors could be reduced if the 180°-pulses in a SE train were phase shifted 90° with respect to the initial 90° pulse. In other words, if the 90°-pulse were applied along the x-axis, the 180°-pulses would be applied alternately along the ±y-axes. This technique, subsequently known by the acronym CPMG (Carr-Purcell-Meiboom-Gill), was extremely robust and is still employed on modern MR imagers when the SE technique is selected.